Thermionically emitting device



Dec. 30, 1958 PAO-HslEN FANG 2,866,915

THERMIONICALLY EMITTING DEVICE Filed Sept. 20, 1956 OIL-sv. uw

HTR/VEY nited States Patent O THERMIONICALLY EMITTING DEVCE Pao-Hsien Fang, Washington, D. C., assignor to Philco Corporation, Philadelphia, Pa., a corporation of Penn- Sylvania Application September 20, 1956, Serial No. 611,070 7 Claims. (Cl. 313-337) The present invention relates to devices for the thermionic emission of electrons, and particularly to an improved emitter suitable for use in vacuum tube devices and similar apparatus.

`Many important electronic devices now known and widely-used rely for their successful operation upon the thermionic emission of electrons. Among the more common of such devices are the ordinary vacuum-tubev diode and triode, as well as the cathode-ray and magnetron tube. In most of such devices the emitted electrons are attracted by a positively-biased element, and the emitter, because of its relative negative potential, is commonly referred to as a cathode. In such emitters, it is generally desirable that relatively low temperatures be capable of producing high emission current densities, thereby to minimize the deteriorating effects inherent in high temperaturevoperation and to reduce the magnitude of the heating currents required. It is also highly desirable in many applications that the emissive properties of the cathode persist to a substantial degree over long periods of time, and that these properties be substantially uniform over the whole area of the emitter surface.

Among the known types of thermionic emitters are those of metals such as tungsten, in which substantial emission is obtained only at very high temperatures, e. g. 2,000 centigrade or more. Activated metal emitters such as the thoriated-tungsten cathode, and various types of boride cathodes are also known which produce substantial emission at somewhat lower temperatures, but still at temperatures which are undesirably high for many purposes. Perhaps the best previously-known cathode from the viewpoint of low-temperature emission has been the so-called oxide-coated cathode, which produces satisfactory emission at temperatures in the region of 700 C. Unfortunately the success of the operation of the oxide cathodeis based on a delicate balance of the nonstoichiometric composition with a minute excess of the metallic component, which can be attained only through elaborate procedures of preparation and aging. Therefore the oxide cathode is often characterized by unduly short life, caused particularly by poisoning of the emissive coating by minute traces of contaminants. In addition it is fragile under electronic and ionic bombardment such as is encountered in magnetron operation.

There has therefore existed a need in the art for a thermionic emitter which would exhibit substantial emission'at relatively low temperatures, which would be characterized by long life and stability of operation, and yet be simple to manufacture. In an attempt to meet these requirements, there has been developed still another type of cathode commonly known as the dispenser-type cathde, which generally has longer life but requires higher temperatures than the oxide-coated cathode. However, the dispenser-type cathode is relatively complex in structure and critical in constituency and arrangement, and is also often characterized by a lack of uniformity in the emission from various portions of the cathode surface.

Accordingly it is a' object of my invention to provide a new and improved thermionic emitter.

Another object is to provide a thermionic emitter characterized by relatively high emission at relatively low temperatures. Y

A further object is to provide such an emitter which is also characterized by long operating life.

Still another object is to provide a new and improved thermionic cathode which is capable of producing relatively high emission at relatively low temperatures, is characterized by long life, and yet issimple in construction.

It is another object to provide an improved emitter of the above-described general type in which substantially uniform emission may be obtained from the entire cathode area.

The above objects are achieved by the provision of a thermionically-emissive element comprising a singlecrystal of the chemical composition BaTiO3 or PbTiO3. I have found that either of these materials, when heated to temperatures of about 700 to 800 centigrade, exhibits relatively strong emission of electrons from substantially all portions of its exposed surfaces. In addition, the emitter of my invention does no-t appear to be susceptible to poisoningj or to be sensitive tol ion bombardment, and is therefore also characterized by extremely longlife.

The use of a single-crystalline body of these titanate materials, .as opposed to a polycrystalline body containing a conglomerate mixture of minute crystals, has been found to be of great impor-tance in obtaining the desired high emission, since the single-crystalline materials exhibit a degree of emission which is commonly about four orders of magnitude greater than that of the same materials in polycrystalline form. Furthermore, the singlecrystalline nature of the material conduces to a high degree of uniformity of emission from variousI portions of the cathode surface.

While all of the theoretical considerations involved in the operation of this novel form of cathode have not yet been thoroughly explored, it has been confirmed that these titanate materials exhibit semiconductive propertiesV at elevated temperatures of operation, a factor which appears to contribute to the above-mentioned relatively high emission. The extremely high dielectric constant of the material, and its ability to form dipole layers at the surfaces thereof, also appear to be significant in producingthe desired operation.

Other objects and features of the invention will be more fully appreciated from a consideration of the following detailed description, taken in connection with the accompanying drawings, in which:

Figure l is an elevational view, partly in section, illustrating vacuum-tube diode device utilizing the novel thermionic emitter of the invention; and

Figure 2 is a graphical representation illustrating the degree of emission obtained from the emitter of the invention in one of its forms, in comparison to the emission produced by certain other cathodes of the` prior art.

Referring now particularly to Figure 1, in which the several elements are not necessarily to the same scale, there is shown one of the forms of vacuum-tube diode which was fabricated in the course of evaluation of the thermionic emitter of the invention. It will be' understood that While the device is a diode, the emitter of the invention is of general utility as a cathode, and for example may be utilized in triode devices, and particularly in cathode-ray tubes. In Figure l in the single-crystalline titanate emitter element 10 of BaTiO3 or PbTiOa is positioned against the inner, bottom surface of a generally cylindrical, cup-shaped metallic member 12, so as to overlie a circular opening 14 provided in support mem- 16 and 18 passing throughinsulating base mglrnbef. 29,:by..

way of suitable glasstoanetal seals,I to connecting pins 22 and 24.

The apparatus utilized in this instance to produce heat;l

ing` of emitter elementA 10, comprises a` filament '31) located within a secondL metallic cup7lil e` member .32 andv having its two terminal-leads connected to connecting pins 34 and 36,` Member, 32 isheldpressed against the under-surface of" emitter, element 10 by means ofl the centrally-apertured, ceramic disc 318` and` the retaining member 40, which is of a suitable resilient metallic matel rial spot-welded to member A metallic anode element 42 is mounted by pins 44 and 46. from a pair ofk glass supporting rods48 and 50, which is turn are rigidly fixed to cup-shaped member 12 by pins 52nand 54. A n appropriate connection 56 is also provided between anode 42 and the anode cap 58.

inthe` formfotthedevice shown` in Figure '1, a glass envelope"y 60` Ysurrounds the operativen elements just "descibedandis evacuated inthe manner usual for vacuum tubes soas to permit the free owof electrons therein. Furthermore it will be understood that, in the course of assembly of the complete tube, care should be taken to insure achigh degreeof` cleanliness and toy avoid the introduction of harmful contaminants into the device.

In one specific form of the device shown in Figure 1, the emitter element was ofBaTiO3, the aperture 14 was ne-.eighth'- centimeter in diameter and the spacing between anode 4,2l and emitter element was substantially one centimeten.

In norrnaluseasource of filament voltage `is-applied betweenconnceting pins- 3,4 and 56` to heat` the emitter 10 to tlrerequiredA temperature and to permit preliminary agingnthereof.` The desired anode-,cathode voltage, is

thenapplied between capV 58 and either or both otpins` 22and, 2 4. When cap58 is positive with respect to pins 22 and 24, electrons thermionically ernitted by emitter 10 are attractedv to anode42 and ow through the connections andthe external circuitin the usual manner. For the` opposite polarity ofj applied potential, little or no current Hows.` t s As `mentioned hereinbefore, thermionic emitter 10 is a single-crystal of BaTiO3 or PbTiOa.` Methods for producing single-.crystalsof such materials are well known in the art, and hence need notbe described here in detail. For example, f or a detail description of one method of fabricating such single-crystals, reference may be had to U. S. Patent No. 2,736,659. of` E. Lynd andv L.V

Merker, issued February28, 1956.

Referring now toFigurew 2, the emissionobtained froml a BaTiOaHsingle-crystal in accordancerwithrthe invention is shown in comparison with that, of several other known thermionic emitters.` lnthe figure, ordinatesi represent the saturation emission densities for the: severaltypes, o fl emitters, in amperes Per square centimeter: ofsurface,

plotted to a logarithmicY scale, while abscissae represent theA temperature; of the emitterj in degrees. centigrade. Curve Ayrepresents thel emission forartungsten cathode, curve Brepresents thatfor a thoratedtungsten cathode, curve C` shows the emission for an emitter composed of lanthanum hexaboride, while curve D represents the emission obtained with the barium titanate cathode element of the` invention. The curves AB, C were.obtained,hy4

the well-known method in which a pulsed saturation voltage is applied between anode and cathode. The curve D was obtained by applying a D.C. anodecathode voltage of 500 volts, with an anode-cathode spacing of l cm., and by measuring the cathode current produced at various temperatures. From this graph it will be apparent that, while an emission currentfof about 0.1 ampere per square centimeter is obtained with the barium titanate cathode at about"940 C., this same current density is obtained only at susbtantially higher temperatures with the tungsten, thoriated tungsten and lanhanum hexaboride cathodes. More particularly, this current density is obtained in the case of tungsten at about 2100 C., in the case of thoriated-tungsten at about l400 C., and, in the case of the boride emitter, at about l250 C.

Singlecrystal of PbTiO3 have been found to possess substantially the same advantages in thermionic emission asV the barium titanate emitter described hereinabove, andrrnay be employed inthe same structures mentioned with; reference to possible applications of the barium titanate emitter. These advantages include not only the relatively high emission obtained at relatively low temperatures, but. also the long life, uniformity of emission and simplicity of structure characterizing the barium titanate form ofV the invention.

` While the invention has been described with particular referenceto specific embodiments thereof, it may also be embodied in any of a variety of widely diverse forms, such as will occur to those skilled in the art in view of the foregoing description. Accordingly, the scope of the invention is limited only by the appended claims.

Il claim:

1. As a thermionic emitter, a heated body of singlecrystalline, material selected from the group consisting of bariumcmetatianate and lead metatitanate, and an evacuated chamber providing an evacuated region adjacent saidbody.

2,A A s athermionic emitter, aV heated, single-crystalline bodyof BaTiO3 and an evacuated chamber providing an evacuated region adjacent said body.

3. As a thermionic emitter, a heated, single-crystalline body of PbTiO3 and an evacuated chamber providing an evacuated region adjacent said body.

4. Apparatus for producing thermionic emission of electrons, comprising a single-crystalline body of a material selected from the group consisting of BaTiO3 and PbTO3, an evacuated chamber providing an evacuated region adjacent said body, and means for heating said body to an elevated temperature, thereby to produce thermionic emission therefrom.

5. Apparatus in accordance with claim 4, in which saidbody is of BaTiO3.

6. Apparatus in accordance with claim 4, in which said body is ofY PbTiO3.

7. Av vacuum-tube device, comprisingalcathode of ra single-crystalline material selected fromcthe group consisting of BaTiO3 and PbTiO3, and an anode element contained in a common evacuated chamber with said cathode and electrically insulated therefrom.

References Citedrin the file of this patent` UNITED STATES PATENTS 1,541,596 Skaupy etal. June 9, 1925 1,815,779 Koref etal. July 21, 1931 2,686,274 Rooksby Aug. 10, 1954 2,736,659 Lynd et al. Feb. 28, 1956 UNITED STATES PATENT OFFICE CERTII'CATION OF CORRECTION Patent NQQ-ZVSiQlS December 3()xr 1958 Pao-Heilen Fang It is hereby certified that error appears n the above numbered patent requiring correction and that the sd Letters Patent should read as corrected below.

Column v2, line 54, before .-Vacuum-=tuhe=fr insert a column 3, line 19v for "is" read in my line 50q for "detail" read detailed columnla line Il,l for tlanhanum read lanthanum mi; line 16,1 for "crystal" read crystals --g line 34Y after "crystalline" strike out thecomma; Line 35, for V"metat"lattante" read e metattane-te nm.

Signed and sealed this 29th-day of August 1961.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents 

7. A VACUUM-TUBE DEVICE, COMPRISING A CATHODE OF A SINGLE-CRYSTALLINE MATERIAL SELECTED FROM THE GROUP CONSISTING OF BATIO3 AND PBTIO3, AND AN ANODE ELEMENT CONTAINED IN A COMMON EVACUATED CHAMBER WITH SAID CATHODE AND ELECTRICALLY INSULATED THEREFROM. 